In machine tools, tool holders, which may hold tools or laser processing heads, for example, are moved in a specific axis direction by means of drives and, in some cases, intervening mechanical components such as gears or portals to which the tool holder or the laser processing head can be fixed. This axis direction is often referred to as the tool axis or simply the axis. Alternatively or additionally, it is possible to move a workpiece by means of a drive in a specific axis direction. This is often abbreviated to workpiece axis or simply to axis. Since the processing result corresponds to the relative movement between tool and workpiece, it is not important whether the tool or the workpiece is moved.
If two drives are present which act in the same axis direction, then one drive, which often has lower dynamics, is typically referred to as the main or fundamental axis and the second drive, which often has higher dynamics, is typically referred to as the additional axis. A further frequently used term is “basic axis.” A basic axis can comprise one or more fundamental axes, with a relatively low dynamics, acting in a resultant overall direction.
Basic axes on machine tools are often very large, because they have to move other axes and built-on accessories. This leads to large moved masses and low mechanical characteristic frequencies. The dynamics of these basic axes are therefore very limited, both with regard to the achievable speeds and accelerations, and with regard to control circuit dynamics and thus the dynamic axis accuracy. On the other hand, the basic axes enable large traverse paths. If these basic axes are used in order to keep the distance of a processing head from a workpiece constant, this distance control exhibits a very limited control circuit dynamics.
If a short-stroke additional axis is fitted close to the so-called tool center point (TCP) in order to control the distance from the workpiece, this axis has to move only relatively small masses and can therefore perform rapid movements and achieve high control circuit dynamics. On machines with, for example, five degrees of freedom, a position change of the additional axis, which changes the distance of the TCP from a workpiece, acts like a variable tool length. This has an unfavorable influence on the path accuracy of the machine.
U.S. Pat. No. 5,801,939 discloses an apparatus with a device for rough positioning (basic axis) and a device for fine positioning (additional axis). A summing element adds up the movements caused by the two devices. A detection element generates a signal describing the overall movement. The signal is compared with a setpoint value and the deviation (control error of the axis assembly) is transmitted as an input signal to the two devices. The actual value of the additional axis is not measured. Instead, the control error of the axis assembly plus a proportion of the simulated additional axis position is transmitted to the basic axis.
U.S. Pat. No. 5,109,148 describes a positioning device, which comprises two independent drives for adjusting the relative position between a tool and a workpiece. For this purpose, the device comprises two position control circuits independent of one another. The first control circuit for the lower-dynamic drive (basic axis) receives bandwidth-limited positional setpoint values via a filter and with these roughly adjusts the desired position (relative position), i.e., within the scope of its limited dynamics. The positioning components which the basic axis cannot perform dynamically are transmitted to the second control circuit (additional axis).